The evolution of human life history is a complex field with several theoretical pitfalls (cf. Nettle & Frankenhuis, Reference Nettle and Frankenhuis2020; Galipaud & Kokko, Reference Galipaud and Kokko2020; Sear, Reference Sear2020). The authors of the target article do an excellent job in disentangling many of the mechanisms underlying the development of human life-history traits and integrating two popular theoretical predictions in the context of extrinsic mortality – the energetic and the developmental hypotheses. The proposed two-tier model helps to make sense of otherwise puzzling results, such as opposing effects of starvation on the reproductive strategies of males and females that I found in my own research (Borgstede & Scheunpflug, Reference Borgstede and Scheunpflug2024). Nevertheless, in my opinion, the authors should have stressed the hierarchical nature of the two processes more. Life-history theory is concerned with species-level and population-level adaptations – which is in line with the energetic hypothesis. However, it says little about variation between individuals within a population – as implied by the developmental hypothesis. Although predictions derived from life-history theory are frequently generalized to account for individual-level variation, conflating these different aggregate levels may actually yield wrong conclusions.

For example, Williams (Reference Williams1957) proposed that a high extrinsic mortality results in a selection pressure toward an earlier onset of reproduction, because individuals who delay their reproduction will, on average, produce fewer offspring than individuals who start reproducing early. The specific prediction thus derived is that species that face a high extrinsic mortality will, on average, start reproduction earlier in their life than species that face a low extrinsic mortality. However, the above reasoning does not imply that individuals should differentially adjust their reproductive schedules to variation in external mortality. This so-called developmental hypothesis is a prediction about individual responses to a variable environment, not a prediction about population responses (i.e., adjustments of aggregate-level traits such as average age of first reproduction). Generalizing from the population-level to the individual-level, therefore, constitutes a mereological fallacy. Although the authors of the target article acknowledge that the developmental hypothesis “is not mathematically derived” from life-history theory, I think they should have stressed the issue of different levels of adaptations. In my opinion, the problem is not mathematical in nature, but concerns the logic of deduction. Evolutionary hypotheses necessarily build on between-individual variation. However, they ultimately apply to populations. In other words, natural selection requires variation between individuals, but its effects manifest on the level of population averages (cf. Price, Reference Price1970).

Of course, certain within-individual adaptations may well follow the same patterns as corresponding population-level adaptations. For example, the ideal free distribution predicts that the ratio of individuals found at two different habitats should equal the ratio of resources that can be exploited at the two habitats (Fretwell & Lucas, Reference Fretwell and Lucas1969). In fact, the very same relation can be found at the level of individual behavior – the so-called matching law (Herrnstein, Reference Herrnstein1961). Given two choice options, individuals frequently learn to distribute their behavior such that the ratio of responding to the two choice options equals the ratio of obtained reinforcement. However, the fact that the behavior of populations aligns with the behavior of individuals in this particular case does not imply that they are guided by the same mechanism (cf. Kraft & Baum, Reference Kraft and Baum2001). Moreover, there is no a priori argument why individual-level adaptations should align with population-level adaptations. Recent developments in multilevel selection theory suggest that such correspondences between aggregate levels may in fact evolve (see, e.g., Baum, Reference Baum2017; Borgstede, Reference Borgstede2020; Borgstede & Eggert, Reference Borgstede and Eggert2021). However, in general, it does not hold that what is adaptive at the population level must also be adaptive at the individual level.

Although the target article does acknowledge the theoretical gap between population-level and individual-level adaptations, I would have liked to see a more thorough theoretical treatment of the issue. Whereas the authors carefully distinguish between the different levels of analysis in their empirical review, their theoretical analysis of the hierarchical nature of the two tiers remains incomplete. In fact, I doubt that a theoretical integration of the mechanisms involved in the formation of human life histories is possible without at least some amount of formal modeling. For example, formal analysis shows that (contrary to Williams, Reference Williams1957), in an exponentially growing population, the evolutionary advantage of early reproduction is actually independent of external mortality (Day & Abrams, Reference Day and Abrams2020). Therefore, given everything else is equal, there should be no selection pressure for early reproduction if extrinsic mortality increases. The situation changes, however, if population growth is modeled as density-dependent. Depending on the regulating mechanisms of population growth, external mortality may be independent of maturation, favor early reproduction, or even lead to a delayed reproductive onset (De Vries, Galipaud, Kokko, Reference De Vries, Galipaud and Kokko2023). The benefits of formal modeling are further illustrated by recent work on the evolution of predictive adaptive responses. For example, Nettle, Frankenhuis and Rickard (Reference Nettle, Frankenhuis and Rickard2013) show that the selection pressure on individual adjustments to external mortality heavily depends on whether the individual uses an external cue (such as a threatening environment) or an internal state (such as a bad health status) to assess their future reproductive prospects. The point to be made here is that life-history theory is too complex to rely on a heuristic strategy of reasoning. In other words, specific empirical hypotheses require specific formal models.

In conclusion, I think the proposed two-tier model is a valuable contribution to the field. However, in my opinion, the target article does not meet its full potential as an integration of the energetic hypothesis and the developmental hypothesis. I propose that a complete understanding of the two mechanisms requires a more sophisticated (and possibly formal) treatment of the different levels of adaptation involved in the formation of human life histories.